Basically, to generate a pulsed output signal from a periodic ramp signal and a reference voltage, the ramp signal and the reference voltage are applied to inputs of a comparator. The output signal taken from the comparator is the desired pulse signal, the duty cycle of which is steered by adjusting the reference voltage.
Typical prior art switch mode power converters have a supply input and an output that provides a regulated supply voltage. For example, U.S. Pat. No. 5,600,234 shows a DC-DC buck converter with a switching cell that transforms an input voltage to a regulated supply voltage lower than the input voltage. The switching cell is controlled by a pulse signal that has a fixed period and a variable duty cycle. The pulse signal is provided by a summing comparator. The summing comparator has a first differential input pair to which a saw-tooth waveform signal is applied that determines the fixed period of the pulse signal. The duty cycle of the pulse signal is set by a first feedback loop that feeds the a fraction of the voltage at the output of the switching cell to an inverting input of the summing comparator, and by a second feedback loop that includes an integrating differential amplifier. While the first feedback loop ensures a fast transient response, it also introduces a DC error. The second feedback loop has a high gain but slow transient response to correct for the DC error and provide a stable steady-state operating point.
When switched mode converters and class-d amplifiers operate towards minimum duty cycles using a periodic signal with a linear saw-tooth waveform, non-linearity present in the comparator has the effect of increasing the small-signal gain in the control loop. This destabilizes the control loop, resulting in a potential for oscillation. Therefore, small minimum values of duty cycle cannot be achieved with conventional converters.